The Romance of Chemistry 719 



bromine are liquids; while a great number, chiefly metals, are 

 solids, like gold, iron, zinc, etc. 



Professor Meldola says with respect to the distribution of the 

 elements : 



It is of interest to note that more than three-fourths of 

 the accessible crust of this globe upon which we live is made 

 up of two non-metals, oxygen and silicon, about one-half 

 being oxygen. Nothing affords more striking evidence of 

 the marvel of chemical change than the contemplation of this 

 geo-chemical fact, that the superficial "solidity" of the earth 

 is due to the predominance of those mineral constituents into 

 the composition of which gaseous oxygen and the non-metal 

 silicon enter to a preponderating extent. The whole crust 

 of the earth with which geology deals is composed to 

 the extent of more than 99 per cent, of only about twenty 

 out of the eighty-two elements. This will give an idea of the 

 rarity of some of the materials which the chemist has had to 

 deal with. 



Mixtures and Compounds 



There is an obvious but important difference between a 

 mixture like sand plus sugar, like iron dust plus chalk, and a 

 compound like sugar, or like chalk. A mixture is never homo- 

 geneous ; it can be divided into different ingredients. A compound 

 is always homogeneous ; its particles, however fine, are all the same 

 all the same until we begin to break up the 'compound into 

 its components. With water we can dissolve away the sugar from 

 the sand ; with a magnet we can pick away the iron particles from 

 the chalk ; but it is usually necessary to take more thoroughgoing 

 measures to split up a compound. 



It must not be supposed, however, that the distinction be- 

 tween a mixture and a compound is always easy. Air looks very 

 homogeneous, but it is a mixture of a great crowd of oxygen 

 particles, a still greater crowd of nitrogen particles, and small 

 crowds of carbon dioxide and water-vapour particles. Nothing 



